1. Field of the Invention
This invention relates to a wet friction material that generates a torque by applying a high pressure to an opposite surface while being immersed in an oil. In detail, this invention relates to a segment-type friction material made by joining segment pieces made of a friction material substrate on one or both of opposite surfaces of a core plate or a core plate of a flat ring shape along an entire circumference thereof. Each of the friction material substrates is cut into a segment piece shape. Particularly, this invention relates to a segment-type friction material having a big effect on decrease of drag torque.
2. Description of the Related Art
In recent year, a segment-type friction material as a wet friction material is under development in order to improve material yield with resultant cost reduction and to lessen drag torque with resultant high fuel efficiency in a vehicle. The segment-type friction material has a core plate of a flat ring shape and segment pieces. The segment pieces are made by cutting a friction material substrate along the ring shape or a circumferential direction of the core plate. A set of the segment pieces are arranged and joined with an adhesive on a front surface of the core plate side by side along an entire circumference of the core plate with a gap to be an oil groove. Another set of the segment pieces made by cutting the friction material substrate are joined with the adhesive to a rear surface of the core plate, too. Such segment-type friction material is applicable to a friction material clutch device that has a single or plural friction plates and that is used in an automatic transmission, which may be referred to as “AT′” hereafter, of automobiles or a transmission of motorcycles or the like.
As an example, a wet hydraulic clutch is used for an automatic transmission of an automobile or the like. The wet hydraulic clutch has plurality of segment-type friction materials and a plurality of separator plates laid alternately on each other. Then, both the plates are pressedly contacted with each other with a hydraulic pressure to transmit a torque. A lubricating oil or an automatic transmission fluid, which may be referred to as “ATF′” hereafter, is supplied to between both the plates for the purpose of absorbing a frictional heat generated when they are shifted from a disengaged state to an engaged state or preventing abrasion of the friction material or the like.
However, a distance between the segment-type friction material and the separator plate is set small in order to increase response of the hydraulic clutch. Moreover, a total area of an oil passage provided on the segment-type friction material is restricted in order to assure a sufficient torque transmission capacity at the time of clutching or engaging operation of the hydraulic clutch. Consequently, it is hard to discharge the lubricating oil remaining between the segment-type friction material and the separator plate at the time of unclutching or disengaging operation of the hydraulic clutch. As a result, there was a drawback that drag torque was generated due to the lubricating oil by relative rotation of both the plates.
In view of the above, according to an invention described in Japanese Laid Open Patent Publication No. 2002-340071, a wet friction plate as a segment-type friction material has two-tiered segment pieces arranged at an inner circumference and an outer circumference thereof. At least part of an inner peripheral edge of the segment piece is inclined outwardly in a radial direction toward a radial inner end of a radially extending oil passage that faces a lagging side in a rotating direction. Thus, in the publication No. 2002-340071, it is recited that it can guide the lubricating oil effectively to the radial inner end of the radially extending oil passage.
However, in the technique described in the above patent publication No. 2002-340071, the radially extending oil passage is narrow and extends with a same width. In contrast, a test was conducted to show a relation between a groove width ratio (a/b) of an outer opening “a” and an inner opening “b” of the radially extending oil passage and a drag torque reduction rate in the segment-type friction material shown in FIG. 5. Then, it was confirmed that the segment-type friction material only had a torque reduction rate of about 30% in case the groove width ratio (a/b) equaled 1.0 or the oil passage extended with the same width. That is, such segment-type friction material only had a half effect of segment-type friction materials whose groove width ratio (a/b) equaled 3.0 or 4.0. FIG. 5 is a drawing showing a relation between a groove width ratio and a drag torque reduction rate in a segment-type friction material. Moreover, in the technique described in the patent publication No. 2002-340071, the segment pieces cut into small pieces are joined with an adhesive in two tiers at the inner circumference and the outer circumference thereby needing a time and effort in manufacturing. But, none the less, there is a problem that the drag torque is not reduced enough by the lubricating oil.